The invention can also be applied to the injection of other materials, for example metals such as aluminium or brass and hence the invention is not necessarily limited to just the injection of plastics only.
The invention equally can be applied in the case that the moulding equipment needs to be modified.
With conventional injection moulding of plastics, molten plastic is injected, under relatively high pressure, into a clamped closed mould cavity.
With this system of injecting into a closed mould cavity the pressure builds up inside the mould and is dependent on the type of plastic material that is being used.
With this conventional technique and its resulting high pressure there are limitations on the maximum flow paths and material wall thicknesses that can be achieved. This results in turn in in-built stresses and quality loss in the product.
On the other hand there are known techniques in which the mould cavity size is increased by not fully closing the mould itself during the initial injection of the material. Thereafter the mould is closed using an adapted mould closing system on the injection machine.
Such a technique is referred to as (Injection) Compression Moulding which has the main drawback of being a process that has limited control, in contrast to the above referred to conventional technique of injection moulding.
Widely known is also an improved variant of compression moulding whereby moveable internal mould parts called cores (or slides) are retracted to a position that results in an enlargement of the mould cavity at the beginning point of the injection of the material.
Following the injection of the material the moveable cores are pushed forward, via a separate mechanism in the machine, to fully compress the material in the mould.
This is a definite improvement over conventional compression moulding from the point of view of control of the process.
Both processes offer improvements in the flow path to wall thickness ratio as well as reduction in the internal part stress of the finished product.
A known disadvantage of the latter method is that the material flows in an uncontrolled manner into the mould, which is a relatively large open space.
As a result the chances are high that flow front lines are produced in the final product.
Furthermore the plastic cools at the contact areas, in an uncontrolled manner, between which the hotter high flow material then gets pressed.
The resultant formation of such jetting effects is an important drawback in particular with amorphous materials especially when the visual aspect of the product is important.
Uncontrolled crystallisation also results in the formation of undesirable cold fronts in the material that can lead to weaknesses and quality issues in the end product.
Neither can one, in a simple manner, control what internal pressures are needed to compress the fluid plastic melt between these cold fonts to fill the cavity completely and reach the desired wall thickness.
Similarly in this context patent EP 1208955 of the company Intercable SRL publishes injection systems for plastics that specifically involve the use of a mould that contains one or more cylindrical controlled cavities in which the flat side of the mould cavity is closed by virtue of a movable core.
In the start position the core is placed against, or at a very small distance from, the opposite flat side of the cylindrical mould resulting in a cavity with either no or extremely low space.
During the injection of the thermoplastic resin the core, which is held in place via an appropriate counterpressure, is slowly pushed back from its starting position resulting in the cavity space being increased in a controlled and symmetrical fashion until finally the full cavity opening is realised.
This allows for the injection of thick wall cylindrical parts that have a smooth stress free surface, for example cylindrical wine bottle cork type shapes.
A known disadvantage is that the moulded parts are relatively small and thick walled and that the part design is limited to just very simple cylindrical parts.
Equally known from patents NL 7906154 and U.S. Pat. No. 4,185,835 of General Electric Schenectady in New York is a process for the injection of plastics, where the mould comprises a stationary unit with a female and male half, which incorporates a moveable compression core.
The mould cavity is defined as the space between the female and male mould halves.
Further the equipment contains the means to inject thermoplastic resin into the mould and the means to control the compression core during the last 20-25% of the injection cycle.
The compression core would be moved by 0.25 mm to 0.50 mm forwards in order to keep the part under a certain compression during the cooling phase.
A well know drawback of this process is that it can only be used to produce parts that are essentially flat or slightly bent such as for example optical lenses for sunglasses, safety glasses and so forth and which preferably have a wall thickness of 3.6 to 3.5 mm.
Further known from patent DE 4301320 of PeguformWerke GmbH, Germany, is a plastics process in which the mould has two halves in between which, in closed form, the cavity is defined and whereby at least one of the mould halves contains one or more moveable cores.
The moveable core(s) can be moved into the cavity and in such a way that they come into contact with the opposite half of the mould.
As a result, in a specific starting condition, the surface of at least one of the mould halves is reduced with the consequence that the mould closing force is lowered.
Following a partial hardening of the material, the cores are retracted from the cavity and newly formed cavities are then filled in a second step under reduced injection pressure.
A known drawback of such a process is that only products with a simple geometry can be produced given that the moveable core can only close onto the opposite form half in a perpendicular way in order to have the effect of lowering the machine's closing force, by reducing the surface area of the mould half
A further drawback of this method is that flow lines and stresses are produced in and around the cavities that are formed following the withdrawal of the moveable cores, which are then filled in a second injection step. These flow lines and stress lines cannot be removed despite various secondary treatments, such as thermal annealing.
In summary one can say that all the above mentioned processes are more machine process linked meaning that it is not possible to have a closed loop process control of the system without taking into account the material properties.